1. Field of the Invention
The invention relates to an electronic memory device, in particular for use in implantable medical appliances.
2. Background Art
Integrated memory devices of this type with one or more electronic read/write memory chips--usually described as RAM (=Random Access Memory)--are basically known in the state of the art and are widely used. For example, these memory devices are used as main memory in personal computers, integrated computer systems, etc.
One or more of these RAM chips are normally held on a board provided with printed conductors. The board is the carrier, in other words is used for the mechanical fixing and bonding of the connections of the RAM chips. The RAM chip itself usually has an activating/deactivating input, a read/write input, a data bus connection and a plurality of address inputs. Further details are not required as they belong to general specialist knowledge.
The present invention relates, in particular, to the field of implantable cardiac pacemakers in which it is already known to use RAM chips. For example, GB-A-20 26 870 describes a microprocessor-controlled cardiac pacemaker in which a RAM chip stores individual stimulation parameters or entire stimulation programs. U.S. Pat. No. 4,223,678 discloses RAM chips as memories for digitalized ECG sequences and specific cardiological events.
With regard to the background of the invention it should be mentioned that, in medical implants such as cardiac pacemakers, the ceramic plates used as substrates which carry RAM chips guarantee high continuous stability and reliability and allow a high packing density in the component set.
The electrical connection between the housing-free components and the conductors of the substrate is produced via so-called bonding connections for which bonding surfaces, so-called bonding pads, are provided on the substrate. An electrical connection between an electrical conductor on the one hand and a connecting leg of an electronic component on the other hand is produced via such bonding pads. This is described as a so-called "die-to-substrate bond".
Alternatively, electronic components and chips can be connected to one another directly via bonding pads and electric lines on the component side. This is described as a so-called "die-to-die bond".
The number of components which can be placed on a common substrate is basically restricted by the surface area of the substrate. The complexity of the conductors, which increases with the number of components, is restricted by the surface area of the substrate as the conductors can be guided only in two dimensions, i.e. without intersections, on the substrate surface. Substrates in which conductors are guided in several mutually separated planes allow complicated remedial action in this respect. The number of components is also restricted by the number of their connecting legs as the necessary bonding pads have to have room on the substrate surface for this purpose.
Starting from this background the following specific drawbacks of the state of the art are to be removed for the field of application of the invention, namely use in implantable medical appliances.
Therefore, when using conventional RAM chips which are generally optimized, in particular with respect to memory capacity and memory access times, in implants, the lack of flexibility in adaptation of the memory capacity and therefore also of the overall volume and the power consumption are drawbacks. On the one hand, along with the design of the contact faces and the electrical connections on the substrate, the size of the attachable RAM chip is also determined inalterably for a specific substrate and, on the other hand, a RAM chip which is adapted with respect to its memory capacity has to be developed specifically for a certain application in each case. Therefore, depending on the number of different applications, a similar number of different RAM chips also has to be developed even though it would be more economical to produce greater numbers of a single memory chip which can be used for various memory requirements.
Furthermore, the connection between RAM chip and the respective substrate, which is normal in the state of the art, is also detrimental because, when using a plurality of RAM chips, a plurality of contact faces which demand a relatively large area is required on the substrate for each individual chip. Furthermore, further components have to be arranged on the substrate for activation/deactivation of the individual chips (address decoders).
Furthermore, when using a plurality of RAM chips, the RAM chips are usually connected indirectly via the substrate so a high number of contacts has to be produced on the substrate. In addition, losses of energy are caused during the transmission of signals into the conductors printed on the substrate owing to the dielectric properties thereof.
A further drawback is that the lithium batteries with a voltage of 2.8 V used in implants do not generally attain the necessary operating voltages for conventional RAM chips.